MK2 RS2000 - Brake Fail Switch info

Hey Guys,
 
Really banging my head against the wall with this. If anyone had any guidance, id really appreciate it.
 
As mentioned, when I measure across the globe holder, I only get about 7-8v. Im not sure if this is normal operation, or if this indicated something wrong. When I turn the accessories on, this is when the power arrives at the globe holder, and when I start the car, nothing changes. The same voltage stays at the globe holder.
 
I figure it must work something like this:
When the accessories are one, the globe must be provided with 12v to light up nice and bright. Then somehow power is removed from one side of it when the car is started.
 
When(if) the piston inside the proportioning valve moves (ie a brake failure), it triggers the switch screwed into the top to again light up the globe.
But I cant seem to figure out how its all wired up.
 
Again - the manuals I have don't seem to shed a lot of light on it.
 
Any help forthcoming would be most welcomed
 
Dave

OK - I think ive made a little headway, but still not fully understanding it.
 
The switch on the top of the differential valve has two pins at the top. These are actually common to each other IE: They are connected. There is a small button on the bottom of the switch which, when depressed, connects to the common rail.
 
I have taken the switch out of the differential valve. If I connect the plug into the top of the switch and then hold the button on the bottom of the pin against an earth, I get ZERO continuity (IE the bulb does not light up). If I then depress the switch against the earth (completing the circuit inside the switch), the bulb lights up nice and bright in the cabin.
 
So, what this tells me is the part of the system that advises of an "actual" brake failure, is working fine. Its just the part that lights the bulb up with accessories that still has me stumped.
 
It must have something to do with the fact that the plug that goes into the top of the switch has two wires. One of then must be a wire back to the bulb, but I don't know what the other one is. In theory if the light did NOT come on with accessories, there would be no need for two wires into the switch. It would only need the one to complete the -ve side of the circuit back to the bulb when the switch was activated.
 
Im getting closer I think, but still need to understand how it is doing the "on with accessories" thing. Obviously once the car is started, it then turns off.
 
Actually one thing I probably should confirm is, am I right in my understanding of the lamp?. I am of the understanding that it is meant to light up when the accessories are turned on, then goes off once the car is started. Can anyone re-confirm for me that this is correct??
 
Dave

hi all,

Just wanted to take a moment to close this thread off with A) the solution, and B) some clarity and correction to some of the above information.

Contrary to what I was previously expecting, the brake failure warning lamp does NOT come on with accessories, it comes on ONLY when the engine is cranking.

Once this was cleared up, obviously this made diagnosis and remedy a great deal easier.

Referring to the above sketch I put together, the line with the question mark next to it simply when to the ignition switch. When the key entering the "cranking" position, the negative side of the circuit is completed, and power is delivered to the bulb in the cabin.

The problem I had was a bung connection between the ignition switch, and the main wiring loom for the car. Once this was fixed, the light started behaving correctly.

Yes - I did need to disconnect the main coil lead, and crank the engine while I took this photo!. But its worth it
 

 
Thanks to everyone for their help. In particular Stewart and Glenn.
 
Dave
 

Thanks Craig.

The bad connector didn't really do anything for the charging issue. That was fixed with a combo of a new battery and another alternator. Whilst there does still seem to be a dull charge light on ( only for the first 5 mins or so then it goes away ).....for the most part it's working and charging.

Thanks for the clarification on the valve name. I think I picked that up from what someone had called it in a previous post..

Getting there!!

Dave

Paul,
 
Thanks for your advice and guidance. Your post was very helpful. Unfortunately my knowledge was not at the level of yours at the time of your post, and so it took me a little bit to catch up.
 
My clarification at the end of my post, was purely so others who have a similar problem in the future can go straight to the end and read the answer in lay terms.
 
Again, thanks very much. Your suggestions were spot on.
 
Dave